The present invention concerns an improved exhaust gas aftertreatment system, and more especially concerns an aftertreatment system for diesel (compression ignition) engines.
Diesel engines are now in widespread use in private cars and light commercial vehicles, as well as in more traditional applications such as buses and trucks, because of their lower fuel consumption than gasoline-fuelled vehicles. Diesel engines operate in a manner that results in an exhaust which still contains significant quantities of oxygen, as well as combustion products and by-products including CO2, H2O, NOx and particulates. There are also minor amounts of unburnt hydrocarbons (HC) and CO present in the exhaust.
Emission regulations for vehicles have been introduced in most countries to improve air quality, particularly in towns and cities. Of the emissions which are regulated, NOx is particularly difficult to treat in diesel exhausts, because of the difficulty in chemically reducing an exhaust gas component in the presence of excess oxygen. Some control of NOx can be achieved by engine design, although usually at the expense of an increase in other pollutants, or by NOx storage on a catalyst component and engine management to provide rich excursions to release stored NOx and to cause chemical reduction of the NOx. State-of-the-art exhaust aftertreatment systems now include a Selective Catalytic Reduction (SCR) stage. SCR involves adding a reductant, usually urea which produces ammonia in use, to reduce NOx to N2 whilst passing the gases over an SCR catalyst. Apart from the additional complexity of on-board storage and supply, and the associated infrastructure, many of the SCR catalysts have rather a narrow temperature window for operation. The on-board storage and supply and the infrastructure issues could possibly be avoided if another reductant, specifically diesel fuel itself, could be used. Such “HC SCR” has been proposed, and suggestions include using zeolites to store unburnt HC for release in an SCR catalyst operating window. Despite its many attractions, HC SCR has proved too difficult to operate in a real-life engine operating situation.
Reforming of hydrocarbons to form synthesis gas (H2 and CO), possibly combined with the water gas shift reaction to increase the yield of H2, is a well-known process practised on an industrial scale.
There have been some suggestions relating to the reforming of fuel, in relation to motor vehicles. Possibly the earliest proposals related to making H2 for the fuelling of fuel cell vehicles. A complication with this is the need to avoid feeding significant quantities of CO into the fuel cell because the Pt catalysts used in the fuel cell are poisoned by CO. More recently, there have been proposals to reform fuel and/or exhaust gases for one or both of two purposes: (a) to recover heat and increase efficiency (with reduced fuel consumption etc) by converting fuel and exhaust components into higher calorific value fuel, with accompanying reduced engine-out emissions; (b) to generate H2 and CO for use in catalytic aftertreatment of exhaust gases to reduce regulated emissions.
Exhaust gases from both major types of internal combustion engine (spark ignition, or gasoline fuelled, and compression ignition or diesel engines) contain high quantities of water vapour, but otherwise vary considerably. Gasoline engine exhausts are high temperature, of the order of 600-800° C., and contain relatively little oxygen. Diesel exhausts are low temperature (sometimes as low as 150° C. in light duty diesels operating under city conditions), and are relatively high in oxygen levels. Low temperatures with diesel exhausts provide challenges for catalysed aftertreatment devices, as the speed of all chemical reactions varies with temperature, and it proves difficult to “light off” the catalyst for all of the different reactions required. The “light off” temperature is considered to be that temperature at which 50% of the reaction takes place.
Unlike large-scale industrial reforming, where temperatures and other parameters such as throughput are controlled, the exhausts from vehicles can vary widely in volume and mass throughput, and in temperature. It has therefore proved very difficult to design and operate an effective diesel aftertreatment incorporating reforming of exhaust gases, although there have been some, essentially academic, proposals.
It is known to produce hydrogen, to improve subsequent aftertreatment emission control, within the engine cylinder itself, by fuel injection and combustion control.
Energy & Fuels 2005, 19, 744-752 discloses a system of exhaust-assisted reforming of diesel fuels. This paper is primarily concerned with recycling reformate to the inlet side of the engine. Although use of the reformate in aftertreatment is mentioned in the introduction, no details of how to achieve this effectively are given.
It is known that hydrogen is effective for the reduction of NO over a Pt-based catalyst at relatively low temperatures (which are representative of gasoline cold-start conditions): J Catalysis 208, 435-447 (2002). It is also known that the addition of hydrogen to a NO/O2/propane mixture assists HC SCR over certain, but not all, silver-based catalysts: see Applied Catalysis B: Environmental 51 (2004) 261-274, U.S. Pat. No. 5,921,076 (Daimler-Benz AG) describes an exhaust system which utilises hydrogen and/or hydrocarbon additions to assist in the reduction of NOx in diesel engine exhausts. Although the possibility of one or more additional catalysts is mentioned, and it is contemplated that such catalyst(s) may store and release hydrocarbons, it is not believed that any embodiment within the scope of the present invention is disclosed or suggested.